The production of elastic laminates from particles of rubber or synthetic resin, using suitable binders or adhesives, is known. Water-permeable, resilient floor coverings for sportsground have been described in German Auslegeschrift No. 1,534,345, but the chemical composition of the binder used is not disclosed. The use of two-component polyurethane binders for binding elastic particles has been described in German Auslegeschrift No. 1,955,267 and in German Offenlegungsschrift Nos. 1,720,059; 2,156,255 and 2,215,893. Moisture hardening one-component polyurethane binders for this field (German Offenlegungsschrift Nos. 2,021,682 and 2,228,111) and for the production of synthetic resin concrete (German Offenlegungsschrift No. 1,931,053) are also known.
The known two-component polyurethane binders are obtained by mixing a polyisocyanate with a polyhydroxyl compound, the two components reacting together in known manner. The polyisocyanate in addition reacts with the moisture from the atmosphere. The proportions in which the two reactions take place depend, among other things, on the atmospheric humidity which affects the properties of the hardened binder in a way which cannot be controlled. This disadvantage is obviated by using one-component polyurethane binders which are hardened only by their reaction with the atmospheric moisture. They consist of prepolymers which contain isocyanate groups and which are obtained from polyhydroxyl compounds and an excess of polyisocyanates. The known one-component polyurethane binders, however, have various disadvantages depending on their chemical structure. Prepolymers based on diphenylmethane-4,4'-diisocyanate crystallize at temperatures around freezing point or above and, therefore, cannot be stored outdoors in cold weather. The subsequent remelting which would be required involves additional expense and may also bring about a chemical change in the prepolymer due to the heat applied. Prepolymers based on commercial diphenylmethane diisocyanate (polyphenylpolymethylenepolyisocyanate) which contains triisocyanates and tetraisocyanates as well as higher functional polyisocyanates of the diphenyl methane series have only a slight tendency to crystallization, but on the other hand their viscosity at room temperature is so high that they are difficult or impossible to process into laminates. In addition, laminates produced in this way have such great hardness and low elongation at break that they cannot be used as coverings for sportsgrounds. Although the tendency to crystallization and the viscosity may be reduced by the addition of low boiling solvents, there is then the risk that the elastic particles may swell due to the absorption of solvents. The solvents are subsequently released in the hardening process so that shrinkage occurs in the elastic layer.
If any arbitrary mixture of tolylene-2,4- and -2,6-diisocyanate is used for preparing the prepolymers, the prepolymers have a relatively high isocyanate vapor pressure at room temperature on account of their free monomeric diisocyanate content. This is physiologically harmful when the prepolymers are being worked-up. On the other hand, removal of the unreacted monomeric diisocyanate involves complicated operations, such as thin layer distillation, quite apart from the fact that NCO prepolymers obtained in this way which are free from monomers have such a high viscosity that they would generally be unusable for the purpose described here. The same applies to the use of hexamethylene-1,6-diisocyanate, but this has the added disadvantage of the relatively low strength of the hardened binder.
It is an object of this invention to obviate the above-described disadvantages of known polyurethane binders.